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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
<title>WebGL - Shadertoy w/mouse</title>
<link type="text/css" href="resources/webgl-tutorials.css" rel="stylesheet" />
</head>
<body>
<canvas id="canvas"></canvas>
</body>
<!--
for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
See https://webglfundamentals.org/webgl/lessons/webgl-boilerplate.html
and https://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
for webgl-utils, m3, m4, and webgl-lessons-ui.
-->
<script src="resources/webgl-utils.js"></script>
<script>
"use strict";

function main() {
  // Get A WebGL context
  /** @type {HTMLCanvasElement} */
  const canvas = document.querySelector("#canvas");
  const gl = canvas.getContext("webgl");
  if (!gl) {
    return;
  }

  const vs = `
    // an attribute will receive data from a buffer
    attribute vec4 a_position;

    // all shaders have a main function
    void main() {

      // gl_Position is a special variable a vertex shader
      // is responsible for setting
      gl_Position = a_position;
    }
  `;

  const fs = `
    precision highp float;

    uniform vec2 u_resolution;
    uniform vec2 u_mouse;

    void main() {
      // gl_FragColor is a special variable a fragment shader
      // is responsible for setting

      gl_FragColor = vec4(fract((gl_FragCoord.xy - u_mouse) / u_resolution), 0, 1);
    }
  `;

  // setup GLSL program
  const program = webglUtils.createProgramFromSources(gl, [vs, fs]);

  // look up where the vertex data needs to go.
  const positionAttributeLocation = gl.getAttribLocation(program, "a_position");

  // look up uniform locations
  const resolutionLocation = gl.getUniformLocation(program, "u_resolution");
  const mouseLocation = gl.getUniformLocation(program, "u_mouse");

  // Create a buffer to put three 2d clip space points in
  const positionBuffer = gl.createBuffer();

  // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  // fill it with a 2 triangles that cover clipspace
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
    -1, -1,  // first triangle
     1, -1,
    -1,  1,
    -1,  1,  // second triangle
     1, -1,
     1,  1,
  ]), gl.STATIC_DRAW);

  let mouseX = 0;
  let mouseY = 0;

  function setMousePosition(e) {
    const rect = canvas.getBoundingClientRect();
    mouseX = e.clientX - rect.left;
    mouseY = rect.height - (e.clientY - rect.top) - 1;  // bottom is 0 in WebGL
    render();
  }

  canvas.addEventListener('mousemove', setMousePosition);
  canvas.addEventListener('touchstart', (e) => {
    e.preventDefault();
  }, {passive: false});
  canvas.addEventListener('touchmove', (e) => {
    e.preventDefault();
    setMousePosition(e.touches[0]);
  }, {passive: false});

  function render() {
    webglUtils.resizeCanvasToDisplaySize(gl.canvas);

    // Tell WebGL how to convert from clip space to pixels
    gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

    // Tell it to use our program (pair of shaders)
    gl.useProgram(program);

    // Turn on the attribute
    gl.enableVertexAttribArray(positionAttributeLocation);

    // Bind the position buffer.
    gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

    // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
    gl.vertexAttribPointer(
        positionAttributeLocation,
        2,          // 2 components per iteration
        gl.FLOAT,   // the data is 32bit floats
        false,      // don't normalize the data
        0,          // 0 = move forward size * sizeof(type) each iteration to get the next position
        0,          // start at the beginning of the buffer
    );

    gl.uniform2f(resolutionLocation, gl.canvas.width, gl.canvas.height);
    gl.uniform2f(mouseLocation, mouseX, mouseY);

    gl.drawArrays(
        gl.TRIANGLES,
        0,     // offset
        6,     // num vertices to process
    );
  }
  render();
}

main();
</script>
</html>


